Resistance change memory

ABSTRACT

According to one embodiment, a resistance change memory includes a memory cell, a sense amplifier and a global bit line. The memory cell is disposed at a location where a local bit line and a word line intersect each other. The memory cell is connected to both the local bit line and the word line. The sense amplifier reads data stored on the memory cell by supplying a read current to the memory cell. The global bit line is connected between the local bit line and the sense amplifier. The global bit line feeds the read current supplied by the sense amplifier to the local bit line. The sense amplifier charges the global bit line, before the local bit line and the global bit line are connected to each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Divisional of U.S. Ser. No. 14/018,242, filed Sep. 4, 2013,which claims the benefit of U.S. Provisional Application No. 61/804,557,filed Mar. 22, 2013, the entire contents of which are incorporatedherein by reference.

FIELD

Embodiments described herein relate generally to a resistance changememory.

BACKGROUND

Recently, semiconductor memories have attracted attention, which includea nonvolatile memory, such as a resistance change memory (e.g.,magnetoresistive random access memory (MRAM), phase change random accessmemory (PRAM), resistive random access memory (ReRAM), etc.) as a memorydevice.

A typical resistance change memory is configured to differentiatebetween data “1” and data “0” by changing its resistance through thesupply of a current (or the application of a voltage). In addition, aresistance change memory is equipped with a sense amplifier that sensesa slight variation in a read current from each memory cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a configuration of a resistance changememory according to a first embodiment;

FIG. 2 is a circuit diagram of a main constituent part of the resistancechange memory;

FIG. 3 is a timing chart of a read operation performed by the resistancechange memory;

FIG. 4 is an exemplary timing chart of a read operation performed by aresistance change memory as a comparative example;

FIG. 5 is a circuit diagram of a main constituent part of a resistancechange memory according to a second embodiment;

FIG. 6 is a timing chart of a read operation performed by the resistancechange memory;

FIG. 7 is a circuit diagram of a main constituent part of a resistancechange memory according to a third embodiment;

FIG. 8 is a circuit diagram of a main constituent part of a resistancechange memory according to a fourth embodiment; and

FIG. 9 is a timing chart of a read operation performed by a resistancechange memory according to a fifth embodiment.

DETAILED DESCRIPTION

Hereinafter, a resistance change memory according to embodiments will bedescribed, with reference to the accompanying drawings. In the followingdescription, the same reference numerals are given to components havingthe same function and configuration, and an overlapping description willbe given only when needed.

In general, according to one embodiment, a resistance change memoryincludes a first memory cell, a word line, a first bit line, first andsecond inverters, and first, second, third, fourth, fifth and sixth MOStransistors. The first memory cell has a resistance change element. Theword line is connected to the first memory cell, and is driven based onan address signal. The first bit line is connected to the first memorycell while intersecting the word line, and is selected based on theaddress signal. The first inverter has a first input terminal, a firstoutput terminal, and first and second voltage terminals. The secondinverter has a second input terminal, a second output terminal, andthird and fourth voltage terminals. The second input terminal isconnected to the first output terminal, and the second output terminalis connected to the first input terminal. The first MOS transistor isconnected to the first output terminal. The second MOS transistor isconnected to the second output terminal. The third MOS transistor isconnected to the first voltage terminal. The fourth MOS transistor isconnected to the third voltage terminal. One end of a current path ofthe fifth MOS transistor is connected to the first voltage terminal. Afirst signal is supplied to a gate of the fifth MOS transistor. A secondbit line is connected to another end of the current path of the fifthMOS transistor. The sixth MOS transistor is connected between the firstand second bit lines. A second signal is supplied to a gate of the sixthMOS transistor. Before the sixth MOS transistor is turned on by thesecond signal, the fifth transistor is turned on by the first signal.

First Embodiment

FIG. 1 is a view illustrating a configuration of a resistance changememory according to a first embodiment.

The resistance change memory includes a memory cell array 11, a senseamplifier 12, drivers/sinkers 13 and 14, a driver 15, a constant currentgeneration circuit 16, a reference current generation circuit 17, and acontroller 18.

The memory cell array 11 has a plurality of memory cells MC arrayed in amatrix fashion. The memory cells MC are connected between a local bitline LBL<0> and a local source line LSL<0>, between a local bit lineLBL<1> and a local source line LSL<1>, . . . and between a local bitline LBL<n> and a local source line LSL<n>, respectively. Furthermore,the memory cells are connected to word lines WL<0> to WL<n>,respectively. In other words, the memory cells MC are arranged atlocations where the word lines WL<0> to WL<n> intersect both the localbit lines LBL<0> to LBL<n> and the local source lines LSL<0> to LSL<n>,respectively. It should be noted that n represents 0, 1, 2, . . . or n.

Ones of the local bit lines LBL<0> to LBL<n> are connected to a globalbit line GBL through N-channel MOS field-effect transistors(hereinafter, referred to as nMOS transistors) M1<0> to M1<n>,respectively. Column selection signals CSL<0> to CSL<n> are supplied tothe gates of the nMOS transistors M1<0> to M1<n>, respectively. Theother end of each of the local bit lines LBL<0> to LBL<n> is connectedto the plurality of memory cells MC.

The global bit line GBL is connected to the driver/sinker 14.Furthermore, the global bit line GBL is connected to the sense amplifier12 through an nMOS transistor M4. The gate of the nMOS transistor M4 isconnected to the constant current generation circuit 16 for generating aconstant current. The global bit line GBL is connected to a referencevoltage terminal, such as a ground potential terminal Vss, through annMOS transistor M6. A discharge signal DIS is supplied to the gate ofthe nMOS transistor M6.

One ends of the local source lines LSL<0> to LSL<n> are connected to aglobal source line GSL through nMOS transistors M2<0> to M2<n>,respectively. The column selection signals CSL<0> to CSL<n> are suppliedto the gates of the nMOS transistors M2<0> to M2<n>, respectively. Theother end of each of the local source lines LSL<0> to LSL<n> isconnected to the plurality of memory cells MC.

The global source line GSL is connected to the driver/sinker 13.Furthermore, the global source line GSL is connected to a referencevoltage end, such as the ground potential terminal Vss, through an nMOStransistor M3. A signal SINK is supplied to the gate of the nMOStransistor M3. Furthermore, the global source line GSL is connected to areference voltage terminal, such as the ground potential terminal Vss,through the nMOS transistor M8. A discharge signal DIS is supplied tothe gate of the nMOS transistor M8.

The drivers/sinkers 13 and 14 feed a write current to each memory cellMC in a direction according to date to be written, during a writeoperation. In this way, the drivers/sinkers 13 and 14 write date intoeach memory cell MC.

The word lines WL<0> to WL<n> are connected to the driver 15 for drivingthe word lines WL<0> to WL<n>.

The gate of the nMOS transistor M4 is connected to the constant currentgeneration circuit 16 for generating a constant current. The senseamplifier 12 is connected to the reference current generation circuit 17for supplying a reference current to the sense amplifier 12.Furthermore, the controller 18 is connected to both the driver 15 andthe sense amplifier 12. The controller 18 controls the operationsperformed by individual parts of the above resistance change memory. Forexample, the controller 18 generates a control signal to be supplied tothe sense amplifier 12, and controls a read operation performed by thesense amplifier 12.

FIG. 2 is a circuit diagram of a configuration of the memory cell array11, the sense amplifier 12 and the constant current generation circuit16 in FIG. 1.

Hereinafter, a configuration of the memory cell array 11 will bedescribed.

The memory cell array 11 has the plurality of memory cells MC arrangedin a matrix fashion at the locations where the word lines WL<0> to WL<n>intersect both the local bit lines LBL<0> to LBL<n> and the local sourcelines LSL<0> to LSL<n>, respectively, as described above. It should benoted that n represents 0, 1, 2, . . . or n.

Each memory cell MC includes, for example, a resistance change elementRE and a selection transistor ST. The resistance change element RE is anelement that is configured to change its resistance through the supplyof a current or the application of a voltage. Examples of the resistancechange element RE include, but are not limited to, a magnetic tunneljunction (MTJ) element, a variable resistive element, a phase changeelement, and a ferro-electric element. The gate of the selectiontransistor ST is connected to the word line WL. The selection transistorST is turned on by the word line WL, whereby the memory cell MC isselected.

Ones of the local bit lines LBL<0> to LBL<n> are connected to the globalbit line GBL through the column selection transistors M1<0> to M1<n>,respectively. The column selection signals CSL<0> to CSL<n> are suppliedto the gates of the column selection transistors M1<0> to M1<n>,respectively.

The global bit line GBL is connected to a connection node between nMOStransistors M12 and M15 in the sense amplifier 12 through a clamptransistor M4 and a transfer transistor M5, both current paths of whichare connected in series to each other. Furthermore, the global bit lineGBL is connected to the reference voltage terminal, such as the groundpotential terminal Vss, through the discharge transistor M6. Thedischarge signal DIS is supplied to the gate of the discharge transistorM6.

One ends of the local source lines LSL<0> to LSL<n> are connected to theglobal source line GSL through the column selection transistors M2<0> toM2<n>, respectively. The column selection signals CSL<0> to CSL<n> aresupplied to the gates of the column selection transistors M2<0> toM2<n>, respectively.

The global source line GSL is connected to the reference voltageterminal, such as the ground potential terminal Vss, through thetransfer transistor M3. The signal SINK is supplied to the gate of thetransfer transistor M3. Furthermore, the global source line GSL isconnected to the reference voltage, such as the ground potentialterminal Vss, through the discharge transistor M8. The discharge signalDIS is supplied to the gate of the discharge transistor M8.

Hereinafter, a configuration of the sense amplifier 12 will bedescribed.

The sense amplifier 12 is a current sensing type of sense amplifier. Thesense amplifier 12 is provided with: a first inverter including aP-channel field-effect transistor (hereinafter, referred to as a pMOStransistor) M11 and an nMOS transistor M12; a second inverter includinga pMOS transistor M13 and an nMOS transistor M14; nMOS transistors M15and M16; and pMOS transistors M17 and M18.

The first inverter (the transistors M11 and M12) includes a first inputterminal, a first output terminal, and first and second voltageterminals. The second inverter (the transistors M13 and M14) includes asecond input terminal, a second output terminal, and third and fourthvoltage terminals. The second input terminal is connected to the firstoutput terminal, and the second output terminal is connected to thefirst input terminal.

The first output terminal of the first inverter is connected to thedrain of the pMOS transistor M17, and the source of the pMOS transistorM17 is connected to a power supply voltage terminal VDD. The secondoutput terminal of the second inverter is connected to the drain of thepMOS transistor M18, and the source of the pMOS transistor M18 isconnected to the power supply voltage terminal VDD. A first sense enablesignal SEN1 from the controller 18 is supplied to both gates of the nMOStransistors M17 and M18.

A first voltage terminal of the first inverter (or the source of thetransistor M12) is connected to the drain of the nMOS transistor M15,and the source of the nMOS transistor M15 is connected to the groundpotential terminal Vss. The third voltage terminal of the secondinverter (or the source of the transistor M14) is connected to the drainof the nMOS transistor M16, and the source of the nMOS transistor M16 isconnected to the ground potential terminal Vss. A second sense enablesignal SEN2 from the controller 18 is supplied to both gates of the nMOStransistors M15 and M16.

The first voltage terminal of the first inverter (or the source of thetransistor M12) is connected to the drain of the nMOS transistor M5. Aread enable signal REN from the controller 18 is supplied to the gate ofthe nMOS transistor M5. The source of the nMOS transistor M5 isconnected to the global bit line GBL through the nMOS transistor M4. Thegate of the nMOS transistor M4 is connected to the constant currentgeneration circuit 16.

The third voltage terminal of the second inverter (or the source of thetransistor M14) is connected to the reference current generation circuit17. The reference current generation circuit 17 is configured to supplya reference current IREF to the sense amplifier 12. Here, the referencecurrent IREF is set to an intermediate value between respective cellcurrents which cause the memory cell to store “0” and “1.” Thisreference current IREF is generated by, for example, a reference cell.

Hereinafter, a configuration of the constant current generation circuit16 will be described.

The gate of the nMOS transistor M4 is connected to the constant currentgeneration circuit 16. The constant current generation circuit 16applies a clamp voltage Vclamp (for example, 0.1 to 0.6 V), which is apreset analog voltage, to the gate of the nMOS transistor M4 during aread operation. In response, a voltage at the drain of the nMOStransistor M4 is clamped constant. This constant voltage serves thepurpose of limiting a current flowing through each memory cell MC so asnot to exceed an upper limit, thus preventing data stored on a selectedone of the memory cells MC from being damaged.

The constant current generation circuit 16 includes a constant currentsource I1, an nMOS transistor M7, and a resistor R1. The drain of thenMOS transistor M7 is connected to the power supply voltage terminal VDDthrough the constant current source I1, and connected to the gate of thenMOS transistor M7 itself. The source of the nMOS transistor M7 isconnected to the ground potential terminal Vss through the resistor R1.

Next, a description will be given of a read operation performed by theresistance change memory according to the first embodiment, withreference to FIG. 3.

FIG. 3 is a timing chart of a read operation performed by the resistancechange memory.

A standby state, which is assumed before a read operation is initiated,is as follows.

In the sense amplifier 12, the first sense enable signal SEN1 is “Low”in level, which causes the pMOS transistors M17 and M18 to be in an onstate, and the second sense enable signal SEN2 is “Low” in level, whichcauses both pMOS transistors M15 and M16 to be in an off state.Moreover, the read enable signal REN is “Low” in level, which causes thenMOS transistor M5 to be in an off state. As a result, both voltagepotentials at nodes SO and SOb are precharged at a potential of a supplyvoltage VDD, and the sense amplifier 12 assumes a standby state.

The clamp voltage Vclamp, which is a constant analog voltage, is appliedto the nMOS transistor M4, and the nMOS transistor M4 is in an on stateduring the standby state.

In the memory cell array 11, the word line WL<n> is in an inactive state(or in a “Low” state) and the column selection signal CSL<n> is in a“Low” state, which causes the nMOS transistor M1<n> to be in an offstate.

After both voltage potentials on the global bit line GBL and the globalsource line GSL are discharged at the grand voltage potential Vss whileboth discharge transistors M6 and M8 are in an on state, both dischargetransistors M6 and M8 are turned off by the discharge signal DIS. Thedischarge transistors M6 and M8 may be optionally provided. Further, thesink transistor M3 in a turn-off state is turned on. It is preferablethat the timing at which the sink transistor M3 is turned on be setbefore a read current flows through the memory cell MC.

In the above state, an external source inputs an active command and anaddress signal to the controller 18 of the resistance change memory. Inresponse, the controller 18 generates a bank active signal foractivating a bank, based on the address signal, and activates a bank tobe used, by using the bank active signal.

At this time, the controller 18 sets the level of the read enable signalREN to “High” through the address signal or the bank active signal,thereby turning on the nMOS transistor M5. As a result, a current flowsfrom the sense amplifier 12 to the global bit line GBL through the nMOStransistors M4 and M5, so that a readout route including the global bitline GBL is charged. It should be noted that the bank active signal isused in the above description, but another internal signal generatedfrom the address signal may be used instead of the bank active signal.

In the memory cell array 11, in turn, the local bit line LBL<n> and thelocal source line LSL<n> are selected through the column selectionsignal CSL<n>. In addition, the word line WL<n> is driven, so that thememory cell MC to be read is selected. Specifically, by setting thelevel of the column selection signal CSL<n> to “High,” the nMOStransistors M1<n> and M2<n> are turned on. As a result, the local bitline LBL<n> is connected to the global bit line GBL, and the localsource line LSL<n> is connected to the global source line GSL.

Furthermore, by setting the level of the word line WL<n> to “High,” theselection transistor ST is turned on. As a result, the memory cell MC tobe read is selected. In this case, the nMOS transistor M3 is in an onstate through the signal SINK, during the read operation. Through theabove processing, a read current is fed from the sense amplifier 12 tothe selected memory cell MC.

Next, by setting the level of the first sense enable signal SEN1 to“High,” both pMOS transistors M17 and M18 are turned off. As a result,precharging both nodes SO and SOb is stopped. Then, the read currentbecomes a current supplied only from the power supply voltage terminalVDD to which both drains of the nMOS transistors M11 and M13 areconnected. In this case, the read current is changed depending on data(“0” or “1”) stored on the selected memory cell MC. In other words, theread current is changed depending on which of the low and highresistance states the selected memory cell MC assumes. The read currentchanged in this manner, namely, depending on the data of the selectedmemory cell MC refers to a cell current IDATA.

After that, by setting the level of the second sense enable signal SEN2to “High,” both nMOS transistors M15 and M16 are turned on. In response,the reference current IREF driven by the reference current generationcircuit 17 is compared with the cell current IDATA. As a result, a levelof a latch circuit including the pMOS transistors M11 and M13 and thenMOS transistors M12 and M14 is retained at a “High” or “Low” level, inaccordance with the comparison result. Finally, (data in) a “High” or“Low” level which is retained in the latch circuit is output from thenodes SO and SOb as output signals OUT and OUTb, respectively.

Here, the reference current IREF is set to an intermediate value betweenrespective cell currents which cause the memory cell to store “0” and“1.” The reference current IREF may be generated by, for example, areference memory cell.

As described above, the nMOS transistor M5 is turned on by the readenable signal REN, thereby charging the global bit line GBL in advance.Then, the memory cell MC is selected by the column selection signalCSL<n> and the word line WL<n>, and the read current is fed to theselected memory cell MC. In other words, the global bit line GBL isprecharged, before the memory cell MC is selected through the columnselection signal CSL<n> and then the word line WL<n> and the readcurrent is fed to the selected memory cell MC. Consequently, it ispossible to shorten a read time by a time required for charging theglobal bit line GBL.

In the first embodiment, after the readout route including the globalbit line GBL is precharged, the read operation is performed by selectingthe memory cell through the column selection signal CSL<n> and feedingthe read current to the selected memory cell. This embodiment enables atime of charging the bit line, which is basically regarded as anunnecessary time and affects the read time, to be removed from an actualread time. Consequently, it is possible to shorten the read time.

Furthermore, by using the address signal or the internal signalgenerated from the address signal, the signal for initiating thecharging of the readout route (read enable signal REN) is generated. Inother words, the address signal or the internal signal generated fromthe address signal triggers the charging of the readout route.Accordingly, it is possible to set the timing of initiating the chargingof the readout route efficiently.

Furthermore, in this embodiment, the bit lines are precharged with thecurrent supplied from the sense amplifier, as described above.Therefore, this embodiment eliminates the need for installing an extraprecharging circuit, and also avoids the increase in the area.

FIG. 4 is an exemplary timing chart of a read operation performed by aresistance change memory as a comparative example.

First, in accordance with input address information, the word line WL<n>is activated and the level of the column selection signal CSL<n> is setto “High,” which turns on the nMOS transistors M1<n> and M2<n>, asillustrated in FIG. 4. Then, the level of the read enable signal REN isset to “High”, which turns on the nMOS transistor M5. In this case, thenMOS transistor M3 is in an on state through the signal SINK, during aread operation.

As a result, a current is fed from the sense amplifier 12 to the globalbit line GBL through the nMOS transistors M4 and M5, so that a readoutroute including the global bit line GBL is discharged and the readcurrent flows through the selected memory cell MC.

In turn, both levels of the first and second sense enable signals SEN1and SEN2 are set to “High.” As a result, a level of the latch circuit isretained at a “High” or “Low” level, in accordance with data stored onthe selected memory cell MC. Finally, (data in) a “High” or “Low” levelwhich is retained in the latch circuit is output.

During the read operation as in FIG. 4, the read enable signal REN isactivated, and then the capacity of the readout route (mainly, theglobal bit line GBL) is charged in accordance with its RC time constant.As a result, a signal appears on the readout route. However, thecharging time for the readout route, as described above, does notdirectly contribute to the operation of reading the memory cell. If thischarging time considerably extends, it may affect the reading time. Inother words, the charging time may extend the reading timeunnecessarily.

In contrast, in the first embodiment, the readout route is charged byactivating the reading start signal (or the read enable signal REN) inadvance through the address signal or the internal signal generated fromthe address signal. Followed by, the word line WL and the columnselection signal CSL are activated, then the initializing signal (or thefirst sense enable signal SEN1) for the sense amplifier is inactivated,and the latch start signal (or the second sense enable signal SEN2) isactivated. Finally, the read current is fed to the selected memory cellMC, and data stored on the selected memory cell MC is read.

In this embodiment, as described above, the readout route including theglobal bit line is precharged, and then the read current is fed to theselected memory cell, so that the read operation is initiated. Thisembodiment makes it possible to shorten the charging time for thereadout route, thereby shortening the reading time. Furthermore, in thisembodiment, the readout route including the global bit line isprecharged with a current supplied from the sense amplifier, asdescribed above. Therefore, this embodiment eliminates the need forinstalling an extra precharging circuit, and also avoids the increase inthe area.

Second Embodiment

As for a second embodiment, a description will be given of an example ofprecharging a global bit line through the switching of a clamptransistor M4.

FIG. 5 is a circuit diagram of a configuration of a memory cell array11, a sense amplifier 12, and a constant current generation circuit 16in the second embodiment.

A resistance change memory according to the second embodiment has acircuit equivalent to that of FIG. 2 from which the transfer transistorM5 disposed between the clamp transistor M4 and the sense amplifier 12is removed, as illustrated in FIG. 5. The remaining configuration of theresistance change memory according to the second embodiment, namely,other configurations of a memory cell array, a sense amplifier, and aconstant current generation circuit are the same as those of the firstembodiment as in FIGS. 1 and 2. Therefore, the descriptions thereof willbe omitted.

First, a description will be given of a read operation performed by theresistance change memory according to the second embodiment, withreference to FIG. 6.

FIG. 6 is a timing chart of a read operation performed by the resistancechange memory.

A standby state, which is assumed before a read operation is initiated,is the same as that as illustrated in FIG. 3.

In the above state, an external source inputs an active command and anaddress signal to a controller 18 of the resistance change memory. Inresponse, the controller 18 generates a bank active signal foractivating a bank, based on the address signal, and activates a bank tobe used, through the bank active signal.

In the above case, the address signal or the bank active signal triggersthe application of a clamp voltage Vclamp to the gate of the nMOStransistor M4. When the clamp voltage Vclamp reaches a predeterminedlevel, the nMOS transistor M4 is turned on. As a result, a current flowsfrom a sense amplifier 12 to a global bit line GBL through the nMOStransistor M4, so that a readout route including the global bit line GBLis charged. It should be noted that the bank active signal is used inthe above description, but another internal signal generated from theaddress signal may be used instead of the bank active signal.

In a memory cell array 11, in turn, a local bit line LBL<n> and a localsource line LSL<n> are selected through a column selection signalCSL<n>. In addition, a word line WL<n> is driven, so that a memory cellMC to be read is selected. Specifically, by setting the level of thecolumn selection signal CSL<n> to “High,” both nMOS transistors M1<n>and M2<n> are turned on. As a result, the local bit line LBL<n> isconnected to the global bit line GBL, and the local source line LSL<n>is connected to a global source line GSL. Furthermore, by setting thelevel of the word line WL<n> to “High,” a selection transistor ST isturned on. As a result, the memory cell MC to be read is selected. Inthis case, an nMOS transistor M3 is in an on state through a signalSINK, during the read operation. Through the above processing, a readcurrent is fed from the sense amplifier 12 to the selected memory cellMC.

Next, by setting the level of a first sense enable signal SEN1 to“High,” both pMOS transistors M17 and M18 are turned off. As a result,precharging both nodes SO and SOb is stopped. Then, the read currentbecomes a current supplied only from a power supply voltage terminal VDDto which both drains of nMOS transistors M11 and M13 are connected. Inthis case, the read current is changed depending on data stored on theselected memory cell MC.

After that, by setting the level of a second sense enable signal SEN2 to“High,” both nMOS transistors M15 and M16 are turned on. In response, areference current IREF driven by the reference current generationcircuit 17 is compared with a cell current IDATA. As a result, a levelof a latch circuit including pMOS transistors M11 and M13 and nMOStransistors M12 and M14 is retained at a “High” or “Low” level, inaccordance with the comparison result. Finally, (data in) a “High” or“Low” level which is retained in the latch circuit is output from thenodes SO and SOb as output signals OUT and OUTb, respectively.

As described above, the nMOS transistor M4 is turned on by the clampvoltage Vclamp, and the global bit line GBL is charged in advance. Then,the memory cell MC is selected by the column selection signal CSL<n> andthe word line WL<n>, and the read current is fed to the selected memorycell MC. In other words, the global bit line GBL is precharged, beforethe memory cell MC is selected through the column selection signalCSL<n> and the word line WL<n> and then the read current is fed to theselected memory cell MC. Consequently, it is possible to shorten a readtime by a time required for charging the global bit line GBL.

In the second embodiment, as described above, after the readout routeincluding the global bit line GBL is precharged, the memory cell isselected by the column selection signal CSL<n>, and the read current isfed to the selected memory cell, so that the read operation isinitiated. This embodiment enables a time of charging the bit line,which is basically regarded as an unnecessary time and affects the readtime, to be removed from an actual read time. Consequently, it ispossible to shorten the read time.

Furthermore, by using the address signal or the internal signalgenerated from the address signal, the supply of a signal for specifyingthe upper limit of a current flowing through the selected memory cell MC(or the application of the clamp voltage Vclamp) is initiated. In otherwords, the address signal or the internal signal generated from theaddress signal triggers the charging of the readout route. Accordingly,it is possible to set the timing of initiating the charging of thereadout route efficiently.

Furthermore, in this embodiment, the bit lines are precharged with thecurrent supplied from the sense amplifier, as described above.Therefore, this embodiment eliminates the need for installing an extraprecharging circuit, and also avoids the increase in the area.

Third Embodiment

In the above first and second embodiments, the resistance change memoryequipped with the current sensing type of sense amplifier has beenprovided. Meanwhile, as for the third embodiment, a description will begiven of a resistance change memory equipped with a voltage sensing typeof sense amplifier.

FIG. 7 is a circuit diagram of a configuration of a memory cell array11, a voltage sensing type of sense amplifier 12A, a constant currentgeneration circuit 16, and a reference voltage generation circuit 17A inthe third embodiment.

Hereinafter, a configuration of a memory cell array 11 will bedescribed.

A global bit line GBL is connected to the gate of an nMOS transistor M15in the sense amplifier 12A through an nMOS transistor (clamp transistor)M4 and an nMOS transistor (transfer transistor) M5, both current pathsof which are connected in series to each other. The remainingconfiguration thereof is the same as that of the memory cell array as inFIG. 2.

Hereinafter, a configuration of the sense amplifier 12A will bedescribed.

The sense amplifier 12A is a voltage sensing type of sense amplifier.The sense amplifier 12A is provided with: a first inverter including apMOS transistor M11 and an nMOS transistor M12; a second inverterincluding a pMOS transistor M13 and an nMOS transistor M14; nMOStransistors M15, M16 and M19; and pMOS transistors M17, M18 and M20.

The first inverter (the transistors M11 and M12) includes a first inputterminal, a first output terminal, and first and second voltageterminals. The second inverter (the transistors M13 and M14) includes asecond input terminal, a second output terminal, and third and fourthvoltage terminals. The second input terminal is connected to the firstoutput terminal, and the second output terminal is connected to thefirst input terminal.

The first output terminal of the first inverter is connected to thedrain of the pMOS transistor M17, and the source of the pMOS transistorM17 is connected to a power supply voltage terminal VDD. The secondoutput terminal of the second inverter is connected to the drain of thepMOS transistor M18, and the source of the pMOS transistor M18 isconnected to the power supply voltage terminal VDD. A first sense enablesignal SEN1 from the controller 18 is supplied to both gates of the nMOStransistors M17 and M18.

The first voltage terminal of the first inverter (or the source of thetransistor M12) is connected to the drain of the nMOS transistor M15.The third voltage terminal of the second inverter (or the source of thetransistor M14) is connected to the drain of the nMOS transistor M16.Both sources of the nMOS transistors M15 and M16 are connected to aground potential terminal Vss through the nMOS transistor M19. A secondsense enable signal SEN2 from the controller 18 is supplied to the gateof the nMOS transistor M19.

The gate of the nMOS transistor M15 is connected to the drain of thenMOS transistor M5. A read enable signal REN from the controller 18 issupplied to the gate of the nMOS transistor M5.

The gate of the nMOS transistor M16 is connected to the referencevoltage generation circuit 17A. The reference voltage generation circuit17A applies a reference voltage VREF to the sense amplifier 12A. Here,the reference voltage VREF is set to an intermediate value betweenrespective cell voltages which cause the memory cell to store “0” and“1.” This reference voltage VREF is generated by, for example, areference cell.

The drain of the nMOS transistor M5 is connected to the power supplyvoltage terminal VDD through the pMOS transistor (load transistor) M20.A load voltage Vload is applied to the gate of the pMOS transistor M20.

Hereinafter, a configuration of the constant current generation circuit16 will be described.

The gate of the nMOS transistor M4 is connected to a constant currentgeneration circuit 16. A configuration of the constant currentgeneration circuit 16 is the same as that of the constant currentgeneration circuit as in FIG. 2.

Next, a description will be given of a read operation performed by theresistance change memory according to the third embodiment.

A timing chart of a read operation performed by the resistance changememory according to the third embodiment is the same as that of FIG. 3.

In the voltage sensing type of sense amplifier 12A, a read current ischanged depending on data stored on the selected memory cell MC, so thata voltage at a connection node between the nMOS transistor M5 and thepMOS transistor M20 is changed. The voltage at the connection node,which is changed in this manner, namely, depending on the data of theselected memory cell MC, refers to a cell voltage VDATA.

In response, the reference voltage VREF applied by the reference voltagegeneration circuit 17A is compared with the cell voltage VDATA. As aresult, a level of a latch circuit including pMOS transistors M11 andM13 and nMOS transistors M12 and M14 is retained at a “High” or “Low”level, in accordance with the comparison result. Finally, (data in) a“High” or “Low” level which is retained in the latch circuit is outputfrom nodes SO and SOb as output signals OUT and OUTb, respectively.

In the third embodiment, similar to the first embodiment, a read enablesignal REN is generated by an address signal or an internal signalgenerated from the address signal, and the nMOS transistor M5 is turnedon by this read enable signal REN. As a result, a global bit line GBL ischarged in advance. Then, the memory cell MC is selected by a columnselection signal CSL<n> and a word line WL<n>, and a read current is fedto the selected memory cell MC. In other words, the global bit line GBLis precharged, before the memory cell MC is selected through the columnselection signal CSL<n> and the word line WL<n> and then the readcurrent is fed to the selected memory cell MC. Consequently, it ispossible to shorten a read time by a time required for charging theglobal bit line GBL.

In the third embodiment, as described above, after the readout routeincluding the global bit line GBL is precharged, the memory cell isselected by the column selection signal CSL<n>, and the read current isfed to the selected memory cell, so that the read operation isinitiated. Accordingly, this embodiment enables a time of charging thebit line, which is basically regarded as an unnecessary time and affectsthe read time, to be removed from an actual read time. Consequently, itis possible to shorten the read time. The remaining configurationthereof is the same as that of the first embodiment as described above.

Fourth Embodiment

In a fourth embodiment, a resistance change memory is equipped with avoltage sensing type of sense amplifier, similar to the thirdembodiment. In the fourth embodiment, a description will be given of anexample of precharging a global bit line through the switching of theclamp transistor M4.

FIG. 8 is a circuit diagram of a configuration of a memory cell array11, a voltage sensing type of sense amplifier 12A, a constant currentgeneration circuit 16, and a reference voltage generation circuit 17A inthe fourth embodiment.

The resistance change memory according to the fourth embodiment has acircuit equivalent to that of FIG. 7 from which the transfer transistorM5 disposed between the clamp transistor M4 and the load transistor M20is removed, as illustrated in FIG. 8. The remaining configuration of theresistance change memory according to the fourth embodiment, namely,other configurations of a memory cell array, a sense amplifier, and aconstant current generation circuit are the same as those of the thirdembodiment as illustrated in FIGS. 1 and 7. Therefore, the descriptionsthereof will be omitted.

Next, a description will be given of a read operation performed by theresistance change memory according to the fourth embodiment.

A timing chart of a read operation performed by the resistance changememory according to the fourth embodiment is the same as that of FIG. 6.

In the voltage sensing type of sense amplifier 12A, a read current ischanged depending on data stored on the selected memory cell MC, so thata voltage at a connection node between an nMOS transistor M4 and a pMOStransistor M20 is changed.

In response, a reference voltage VREF applied by the reference voltagegeneration circuit 17A is compared with a cell voltage VDATA. As aresult, a level of a latch circuit including pMOS transistors M11 andM13 and nMOS transistors M12 and M14 is retained at a “High” or “Low”level, in accordance with the comparison result. Finally, (data in) a“High” or “Low” level which is retained in the latch circuit is outputfrom nodes SO and SOb as output signals OUT and OUTb, respectively.

In the fourth embodiment, similar to the second embodiment, theapplication of the clamp voltage Vclamp is initiated by an addresssignal or an internal signal generated from the address signal, and thenMOS transistor M4 is turned on by this clamp voltage Vclamp. As aresult, the global bit line GBL is charged in advance. Then, the memorycell MC is selected by a column selection signal CSL<n> and a word lineWL<n>, and a read current is fed to the selected memory cell MC. Inother words, the global bit line GBL is precharged, before the memorycell MC is selected through the column selection signal CSL<n> and theword line WL<n> and then the read current is fed to the selected memorycell MC. Consequently, it is possible to shorten a read time by a timerequired for charging the global bit line GBL.

In the fourth embodiment, as described above, after the readout routeincluding the global bit line GBL is precharged, the memory cell isselected by the column selection signal CSL<n>, and the read current isfed to the selected memory cell, so that the read operation isinitiated. This embodiment enables a time of charging the bit line,which is basically regarded as an unnecessary time and affects the readtime, to be removed from an actual read time. Consequently, it ispossible to shorten the read time. The remaining configuration thereofis the same as that of the first embodiment as described above.

Fifth Embodiment

A fifth embodiment provides the same circuit configuration as that ofthe fourth embodiment as in FIG. 8. In this embodiment, a descriptionwill be given of an example of precharging a global bit line through theswitching of a load transistor M20.

A configuration of a resistance change memory according to the fifthembodiment, namely, configurations of a memory cell array, a senseamplifier, and a constant current generation circuit are the same asthose of the fourth embodiment as illustrated in FIGS. 1 and 8.Therefore, the descriptions thereof will be omitted.

Next, a description will be given of a read operation performed by theresistance change memory according to the fifth embodiment, withreference to FIG. 9.

FIG. 9 is a timing chart of a read operation performed by the resistancechange memory.

A standby state, which is assumed before a read operation initiated, isthe same as that as illustrated in FIG. 3.

In the above state, an external source inputs an active command and anaddress signal to a controller 18 of the resistance change memory. Inresponse, the controller 18 generates a bank active signal foractivating a bank, based on the address signal, and activates a bank tobe used, by using the bank active signal.

In this case, the address signal or the bank active signal triggers a“Low” level of the load voltage Vload, thereby turning on the pMOStransistor M20. As a result, a current flows from a power supply voltageterminal VDD to a global bit line GBL through the pMOS transistor M20and an nMOS transistor M4 so that a readout route including the globalbit line GBL is charged. It should be noted that the bank active signalis used in the above description, but another internal signal generatedfrom the address signal may be used instead of the bank active signal.

In a memory cell array 11, in turn, a local bit line LBL<n> and a localsource line LSL<n> are selected through a column selection signalCSL<n>. In addition, a word line WL<n> is driven, so that a memory cellMC to be read is selected. Specifically, by setting the level of thecolumn selection signal CSL<n> to “High,” both nMOS transistors M1<n>and M2<n> are turned on. As a result, the local bit line LBL<n> isconnected to the global bit line GBL, and the local source line LSL<n>is connected to the global source line GSL. Furthermore, by setting thelevel of the word line WL<n> to “High,” a selection transistor ST isturned on. As a result, the memory cell MC to be read is selected. Inthis case, an nMOS transistor M3 is in an on state through a signalSINK, during the read operation. Through the above processing, a readcurrent is fed from a sense amplifier 12A to the selected memory cellMC.

In the voltage sensing type of sense amplifier 12A, the read current ischanged depending on data stored on the selected memory cell MC, so thata voltage at a connection node between the nMOS transistor M4 and thepMOS transistor M20 is changed.

In response, a reference voltage VREF applied by the reference voltagegeneration circuit 17A is compared with a cell voltage VDATA. As aresult, a level of a latch circuit including pMOS transistors M11 andM13 and nMOS transistors M12 and M14 is retained at a “High” or “Low”level, in accordance with the comparison result. Finally, (data in) a“High” or “Low” level which is retained in the latch circuit is outputfrom nodes SO and SOb as output signals OUT and OUTb, respectively.

In the fifth embodiment, the load voltage Vload is generated by anaddress signal or an internal signal generated from the address signal,and the pMOS transistor M20 is turned on by this load voltage Vload. Asa result, the global bit line GBL is charged in advance. Then, thememory cell MC is selected by the column selection signal CSL<n> and theword line WL<n>, and the read current is fed to the selected memory cellMC. In other words, the global bit line GBL is precharged, before thememory cell MC is selected through the column selection signal CSL<n>and the word line WL<n> and then the read current is fed to the selectedmemory cell MC. Consequently, it is possible to shorten a read time by atime required for charging the global bit line GBL.

In the fifth embodiment, as described above, after the readout routeincluding the global bit line GBL is precharged, the memory cell isselected by the column selection signal CSL<n>, and the read current isfed to the selected memory cell, so that the read operation isinitiated. Accordingly, this embodiment enables a time of charging thebit line, which is basically regarded as an unnecessary time and affectsthe read time, to be removed from an actual read time. Consequently, itis possible to shorten the read time. The remaining configurationthereof is the same as that of the first embodiment as described above.

Effect

The above embodiments are applicable to semiconductor memories intowhich data is written with a current, including MRAMs havingmagnetoresistive effect elements, ReRAMs having variable resistiveelements, and PRAMS having phase change elements.

For example, an MRAM includes a magnetic resistive element called amagnetic tunnel junction (MTJ) element, as a memory element. Such an MTJelement includes a fixed layer (or a reference layer), a recording layer(or a free layer), and an insulating layer that is sandwichedtherebetween. In the fixed layer, a magnetization direction is fixed byan antiferromagnetic layer, and in the recording layer, a magnetizationdirection can be inverted freely. The MTJ element utilizes the change inthe resistance in the recording layer in the magnetization direction,relative to the resistance of the fixed layer, which is called amagnetoresistive effect. Thus, the MTJ element differentiates betweendata “1” and data “0” by utilizing a relative difference betweenrespective resistances in the magnetization direction.

A mechanism for writing data into an MRAM, such as a spin injection typeof MRAM, operates as follows. When data “1” is written into the MRAM, acurrent is fed thereto in a direction from the fixed layer to therecording layer of the MTJ element. Meanwhile, when data “0” is writteninto the MRAM, a current is fed thereto in a direction from therecording layer to the fixed layer of the MTJ element.

In the embodiments, as described above, the readout route (mainly, theglobal bit line) is precharged by the address signal or the internalsignal generated from the address signal, before the column selectionsignal or the word line is activated. This is followed by initiation ofthe read operation. Accordingly, these embodiments enable a time ofcharging the readout route, which is basically regarded as anunnecessary time and affects the read time, to be removed from an actualread time. Consequently, it is possible to shorten the read time duringwhich the memory cell array is activated and data is read therefrom.

Furthermore, in accordance with the address signal or the internalsignal generated from the address signal, the signal for initiating thecharging of the readout route is generated. In other words, the addresssignal or the internal signal generated from the address signal triggersthe charging of the readout route. Accordingly, it is possible to setthe timing of initiating the charging of the readout route efficiently.

Furthermore, in the above embodiments, the bit lines are precharged withthe current supplied from the sense amplifier, as described above.Therefore, these embodiments eliminate the need for installing an extraprecharging circuit, and also avoid the increase in the area.

The overall configuration of the resistance change memory according toeach embodiment, namely, the configurations of the memory cell array,memory cells, sense amplifier, drivers/sinkers, constant currentgeneration circuit, reference current generation circuit, and the likeare not limited to those of the above examples. For example, thestructure disclosed by U.S. Pat. No. 7,649,792 or US 2012/0286339 may beemployed. The contents of these specifications are entirely incorporatedherein by reference.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A resistance change memory comprising: a memorycell disposed at a location where a local bit line and a word lineintersect each other, the memory cell having a resistance changeelement, and the memory cell being connected to both the local bit lineand the word line; a sense amplifier that reads data stored on thememory cell by supplying a read current to the memory cell; and a globalbit line connected between the local bit line and the sense amplifier,the global bit line feeding the read current supplied by the senseamplifier to the local bit line, wherein the sense amplifier charges theglobal bit line, before the local bit line and the global bit line areconnected to each other, in response to a read enable signal beingreceived at a gate of a transfer transistor electrically connectedbetween the memory cell and the sense amplifier, and wherein the globalbit line is charged by activating the read enable signal.
 2. Theresistance change memory according to claim 1, wherein the senseamplifier initiates the charging of the global bit line in response toone of (i) an active command, (ii) an address signal that designates thelocal bit line and the word line or a region containing the local bitline and the word line, and (iii) a signal generated from the addresssignal.
 3. The resistance change memory according to claim 1, whereinthe local bit line is coupled to the global bit line by activating acolumn selection signal.
 4. The resistance change memory according toclaim 1, wherein the transfer transistor is directly connected to aninverter of the sense amplifier.
 5. The resistance change memoryaccording to claim 1, wherein a current of the transfer transistor flowsbetween an inverter of the sense amplifier and the global bit line.